Electronic amplifier circuit

ABSTRACT

An amplifier circuit contains a first and second current branch, each branch comprising an input transistor and a cascode transistor. Inputs of the amplifier are coupled to control electrodes of the input transistors in respective ones of the current branches. The control electrodes of the cascode transistors are coupled to each other. A high frequency coupling is provided between the control electrodes of the cascode transistors and a node of the common current source, to copy substantially common mode voltage changes of terminals of the main current channels of the input transistors to voltage changes at the control electrodes of the cascode transistors. Thus, changes in the voltage difference between the voltages at the different terminals of each input transistor are substantially eliminated, preventing parasitic currents from the control electrode of the input transistor to the main current channel of the cascode transistors, that would reduce the common mode rejection ratio and thereby linearity of the amplifier.

The invention relates to an electronic amplifier circuit.

PCT patent application WO 01/26216 discloses a differential amplifiercircuit. This amplifier circuit has differential input connections anddifferential output connections. The amplifier circuit contains twobranches, each with a series connection of the main current channels ofan input transistor and a cascode transistor coupled between a commoncircuit and a respective one of the differential output connections. Thedifferential input connections are coupled to the control electrodes ofthe input transistors in the two branches.

High demands are placed on the linearity of such an amplifier, when itis used as a broadband amplifier to amplify signals from a band thatcontains many signals of comparable strength, such as may televisionchannels in a Cable TV signal. WO 01/26126 describes how the linearitycan be improved by ensuring a high transconductance of the amplifier athigh frequencies, through a coupling that places the input transistorand the cascode transistor in each branch in parallel at higherfrequencies.

It has been found that the common mode rejection of broadband amplifiersis an important factor in determining the non-linearity. This is becausethe even order intermodulation products of differential signalsgenerated by the transistors are common mode signals. These even orderintermodulation products include second order intermodulation products,which are potentially the strongest intermodulation products and whichare particularly relevant for distortion in broadband amplifiers. Bymaximizing the common mode rejection of the differential amplifier thecontribution of these even order intermodulation products to the outputis minimized.

Amongst others, it is an object of the invention to improve thelinearity of the differential amplifier.

Amongst others, it is an object of the invention to increase the commonmode rejection of the differential amplifier.

The amplifier according to the invention is set forth in claim 1. Byproviding a high frequency coupling between a common point of thebranches and the control electrodes of the cascode transistors of thebranches it is ensured that there is little or no common mode voltagedifference between the terminals of the main current channel of eachinput transistor and its control electrode. As a result of this, thecircuit eliminates the effect on the gain exerted by the capacitancebetween the control electrode of the input transistor in a branch andthe node that connects the main current channels of the input transistorand the cascode transistor in that branch (mainly the Miller capacitancein case of bipolar input transistors). This increases common moderejection and thereby increases linearity.

The common point of the branches is fed by a common current sourcecircuit. This circuit supplies the currents to the branches in common.Ideally this common current source circuit should be a perfect currentsource, producing a current that is independent of the voltage acrossthe current source circuit, at least in response to voltage changes inthe amplified frequency band. But it has been found that the amplifierhas good linearity even if the current source circuit has a behaviourthat differs far from the behaviour of such a perfect current source.

An embodiment of the amplifier according to the invention comprises a DCbias supply circuit coupled to the control electrodes of the cascodetransistors of the branches, the DC bias supply circuit comprising ahigh frequency blocking circuit. Thus, it is ensured that high frequencycurrent to the cascode transistors has to flow to the control electrodesof these transistors, forcing the cascode transistors to minimize thecommon mode voltage differences over the input transistors.

These and other objects and advantageous aspects of the amplifieraccording to the invention will be described in more detail using thefollowing figures.

FIG. 1 shows a differential amplifier

FIG. 2 shows non-linearity as a function of frequency

FIG. 1 shows a differential amplifier. The amplifier contains twobranches 10, 11. Each branch contains a series connection of the maincurrent channels of an input transistor 100, 110 and a cascodetransistor 104, 114 connected between a common circuit 120, 122, 124 andan output transformer 160. The control electrodes of the inputtransistors 100, 110 are coupled to an input transformer 140. Althoughsymmetrical inputs and outputs are shown, it will be understood thatusually one terminal of each of the input transformer and the outputtransformer is connected to ground, so as to create an asymmetric inputand output. Preferably, the input transistors 100, 110 of both brancheshave substantially mutually equal parameters. Preferably also cascodetransistors 104, 114 have substantially mutually equal parameters.

By way of example bipolar transistors 100, 104, 110, 114 are shown. Thecommon circuit contains a common node 126, connected to the emitters ofthe input transistors 100, 110 via respective emitter resistances 122,124 and to ground via a ground impedance 120. A power supply connectionV is coupled to the collectors of the cascode transistors 104, 114. Thecontrol electrodes of the input transistors 100, 110 are biased with abase voltage Vb via the input transformer 140. Feedback impedances 106,116 couple the outputs of the branches 10, 11 to the control electrodesof the input transistors 100, 110.

The control electrodes of the cascode transistors 104, 114 are coupledto each other. A capacitor 18 couples the common node 126 is connectedto the control electrodes of the cascode transistors 104, 114. Thecontrol electrodes of the cascode transistors 104, 114 are biased viabias network 192, 194, 190.

In operation, an input voltage difference is applied between the controlelectrodes of the input transistors 100, 110 from input transformer 140.This results in a current difference between the currents flowingthrough the main current channels of the input transistors 100, 110.Common circuit 120, 122, 124 serves as a current source at least in thefrequency range for which the amplifier is used. For this purpose,ground impedance 120 has a relatively high value, at least for higherfrequencies. This may be realized by using an inductance in groundimpedance 120, or by means of a current source circuit in groundimpedance.

The currents from the common circuit 120, 122, 124 mainly flow from themain current channels of the input transistors 100, 110 through the maincurrent channels of the cascode transistors 104, 114 to the outputtransformer 160. Cascode transistors 104, 114 serve to provide a lowimpedance at nodes 102, 112 in the branches 10, 11 between the maincurrent channels of the input transistors 100, 110 and the cascodetransistors 104, 114. This low impedance reduces undesirable feedbackeffects of the input transistors 100, 110. Feedback impedances 106, 116are optionally used to adjust the frequency dependence of the amplifiergain and the input impedance at the control electrodes of the inputtransistors 100, 110, so as to provide a matched input impedance at theinput of the amplifier. The difference between the currents leads to anoutput voltage at the output of the output transformer 160.

Because of the current source behaviour of the common circuit 120, 122,124 the sum of the currents from the common circuit 120, 122, 124 to themain current channels in both branches 10, 11 is a constant independentof the input signal, at least in a frequency band of interest. Absentinjection of further current from outside the branches 10, 11, allcurrents though the branches 10, 11 are proportional to these currentsfrom the common circuit 120, 122, 124 and therefore the sum of any pairof corresponding currents from the branches 10, 11 is a constantindependent of the input signal.

The currents from the common circuit 120, 122, 124 mainly flow from themain current channels of the input transistors 100, 110 through the maincurrent channels of the cascode transistors 104, 114 to the outputtransformer 160. The difference between the currents leads to an outputvoltage at the output of the output transformer 160. Ideally, the commonmode component (sum) of the currents cannot contribute to the outputvoltage if the output transformer 160 is ideally symmetric. Thiscorresponds to an infinite common mode rejection ratio. But in practice,due to the inevitable asymmetries the common mode component may lead toan output signal.

Non-linearity, in particular second order intermodulation effectscontribute to the output signal through the generation of a common modecomponent. This component differs significantly from zero only to theextent that input dependent common mode currents from other sources thanthe high impedance common circuit 120, 122, 124 flow into the branches10, 11. A relevant current of this type is the common current throughthe base collector capacitance of the input transistors 100, 110.

Capacitor 18 serves to provide a high frequency coupling between thecontrol electrodes of cascode transistors 104, 114 and common node 126of common circuit 122, 124, 120. The value of capacitor 18 is selectedat least so large that in the frequency band for which the amplifier isused the common mode voltage changes of the emitters of the inputtransistors 100, 110 are substantially copied to the control electrodesof the cascode transistors 104, 114. A surprisingly low capacitancevalue suffices for this purpose, because the impedance at the controlelectrodes of cascode transistors 104, 114 is relatively high. Thismakes it possible to integrate capacitance value in an integratedcircuit.

As a result of capacitance 18 and the effect of cascode transistors 104,114 the common mode voltage changes are also copied to the nodes 102,112 between the main current channels of the cascode transistors 104,114 and the input transistors 100, 110. Thus, the common mode part ofthe voltage at the collectors of the input transistors 100, 110 issubstantially equal to the common mode part of the voltage at theiremitters, which in turn is substantially equal to the common mode partof the voltage at the control electrodes of the input transistors 100,110, because of the high impedance of the common ground impedance 120 atthe relevant frequencies.

As a result of the equal common mode voltages substantially no commonmode current flows through the base-collector capacitance of the inputtransistors 100, 110 (e.g. through the Miller capacitance). Effectively,the circuit creates a very high impedance circuit for common modevoltages at the control electrodes of the input transistors. As aresult, the amplifier realizes a very high rejection for common modevoltages and intermodulation products are suppressed. (It should benoted that, as a result of capacitor 18 the common mode rejection ratiois very high even if the current source behaviour of common circuit 120,122, 144 is far from perfect).

FIG. 2 shows a graph of the strength of second order distortion as afunction of frequency. The strength is plotted on vertically on alogarithmic scale, to allow for comparison of strengths under differentcircumstances. A top curve shows distortion of the amplifier withoutcapacitor 18. A second curve from above shows distortion with capacitor18 added.

The effect of capacitor 18 can be improved by forcing substantially allcurrent from common node 126 to flow through the control electrodes ofcascode transistors. For this purpose, bias circuit 190, 192, 194preferably contains an inductance 190 that presents a substantiallyhigher impedance to signals at the relevant frequencies than the controlelectrode of cascode transistors 104, 114. A third curve from above inFIG. 2 show the strength of distortion when an inductance 190 has beenadded to the bias circuit when that inductance ideally blocks allrelevant signal currents.

Although the principle of the invention has been illustrated using thecircuit of FIG. 1, it will be appreciated that, without deviating fromthe invention many modifications may be made to the circuit. Forexample, part or all of the bipolar transistors 100, 104, 104, 114 maybe replaced by field effect transistors. Each transistor may be realizedby a parallel arrangement of a plurality of transistors. Such a parallelarrangement has the same effect as a single transistor, and, as usedherein, is considerer to be covered by the word “transistor”. Instead oftransformers 160, 140 other circuits for feeding and/or extractingdifferential input and output signals may be used. Additional componentsmay be added to the branches 10, 11, to the bias circuit 190, 192, 194or the to the common circuit 122, 124, 120. Or components, for exampleemitter resistances 122, 124 may be deleted from the circuit.

Capacitor 18 may be replaced or supplemented by further capacitorsbetween the control electrodes of cascode transistors 104, 114 and nodesin the common circuit 122, 124, 120 that substantially carry thevariations of the emitter voltage of the input transistors 100, 110, sothat a common mode voltage at the common circuit 120, 122, 124 iscoupled to the control electrodes of common mode transistors 104, 114.For example a pair of capacitors (not shown) may be coupled from thecontrol electrodes of the cascode transistors 104, 114 to the nodesbetween emitter resistors 122, 124 and input transistors 100, 110. Inanother example, T-shaped common circuit 120, 122, 124 may be replacedby a Pi shaped circuit, with impedances from the emitters to ground andan impedance between the emitters. In this case capacitors between theemitters of the input transistors 100, 110 and the control electrode ofthe cascode transistors 104, 114 may be used.

Also, although a simple capacitive coupling is preferred, it will beappreciated that any coupling that allows the common mode voltage fromthe common circuit 120, 122, 124 to be copied to the control electrodesof the cascode transistors 104, 114 in the frequency band of interestwill achieve the desired effect of reducing distortion. In onealternative example, a buffer amplifier with a gain of substantially onemay be used with an input coupled to a common point coupled to theemitters of the input transistors 100, 110 and an output coupled to thecontrol electrode of cascode transistors 104, 114.

Emitter resistances 122, 124 are not essential for the effect ofcapacitor 18, but serve to adjust to the input impedances presented bythe base the input transistors 100, 110 so as to match the impedancecoupled to the input transformer, as well as to improve linearity. Inone non-limitative example the emitter resistances have a value of 4Ohm. Feedback impedances are not essential for the effect of capacitor18 either, but serve to adjust the gain of the amplifier (in particularits frequency dependence) and to improve impedance matching. In onenon-limitative example feedback impedances 106, 116 each contain aseries arrangement of a capacitance in arrange of 100–10000 pF, aninductance in a range of 1–50 nH and a resistor in a range of 400 of1500 Ohm. The capacitance serves as a DC-decoupling. The inductanceserves to increase gain at higher frequencies. The resistor similarlyserves to reduce the frequency dependent variation of the gain.

Thus, by adding a coupling between the common point on one hand and thecontrol electrodes of cascode transistors 104, 114 common mode rejectionand linearity of the differential amplifier are improved.

1. An amplifier circuit comprising: input connections; a first andsecond current branch (10, 11), each comprising an input transistor(100, 110) and a cascode transistor (104, 114), the input connectionsbeing coupled to control electrodes of the input transistors (100, 110)in respective ones of the current branches (10, 11), control electrodesof the cascode transistors (104, 114) being coupled to each other; acommon current source circuit (120, 122, 124); output connections, eachcoupled to the common current source circuit (120, 122, 124) via aseries connection of the main current channels of the cascode transistor(104, 114) and the input transistor successively, of a respective one ofthe current branches; a high frequency coupling (18) between the controlelectrodes of the cascode transistors and a node of the common currentsource arranged to copy substantially common mode voltage changes ofterminals of the main current channels of the input transistors tovoltage changes at the control electrodes of the cascode transistors;and a bias circuit coupled to the control electrodes of the cascodetransistors for biasing the control electrodes of the cascodetransistors, the bias circuit comprising a high frequency currentblocking circuit including an inductance for blocking flow of currentfrom the high frequency coupling through the biasing circuit.
 2. Anamplifier circuit according to claim 1, wherein the high frequencycoupling comprises a capacitance coupled between the control electrodesof the cascode transistors and the node of the common current source,with a capacitance value so that common mode voltage changes of theterminals of the main current channels of the input transistors aresubstantially coupled to voltage changes at the control electrodes ofthe cascode transistors.
 3. A wide band high frequency signaldistribution system, containing an amplifier according to claim
 1. 4. Anmethod of amplifying a wide band signal, the method comprising:inputting the wide band signal to a control electrode of inputtransistors in a first and second current branch, each current branchcomprising one of the input transistors and a cascode transistor, theinput connections being coupled to control electrodes of the inputtransistors in respective ones of the current branches, controlelectrodes of the cascode transistors being coupled to each other,outputting amplified signals from output connections that are coupled toa common current source circuit via a series connection of the maincurrent channels of the cascode transistor and the input transistorsuccessively, of a respective one of the current branches; copyingsubstantially common mode voltage changes of terminals of the maincurrent channels of the input transistors to voltage changes at thecontrol electrodes of the cascode transistors, at least in a frequencyband of the wide band signal; biasing the control electrodes of thecascade transistors with a bias circuit; and providing the bias circuitwith a high frequency current blocking circuit including an inductance.5. An amplifier circuit comprising: input connections; a first andsecond current branch, each comprising an input transistor and a cascodetransistor, the input connections being coupled to control electrodes ofthe input transistors in respective ones of the current branches,control electrodes of the cascode transistors being coupled to eachother; a common current source circuit; output connections, each coupledto the common current source circuit via a series connection of the maincurrent channels of the cascode transistor and the input transistorsuccessively, of a respective one of the current branches; and a highfrequency coupling between the control electrodes of the cascodetransistors and a node of the common current source arranged to copysubstantially common mode voltage changes of terminals of the maincurrent channels of the input transistors to voltage changes at thecontrol electrodes of the cascode transistors; wherein a feedbackimpedance element is coupled between an output of at least one of thefirst and the second current branch and the control electrode of theresponsive input transistor.